1. Field of the Invention
This invention pertains to synchronous shifting multirange hydromechanical steering transmissions for tracklaying or skid-steered wheeled vehicles having hydraulic components to provide continuously variable hydrostatic drive ratios and mechanical components wherein selective operation of clutches or brakes causes the transmission to operate in a plurality of distinct hydrostatic, hydromechanical or mechanical ranges and wherein steering is effected by using one or more hydrostatic devices to impose a speed differential or difference between left and right outputs. More specifically, this invention pertains to such transmissions having internal integral steering resulting from use of the same hydrostatic components for both drive and steer rather than from use of dedicated hydrostatic components solely to impose a difference of speed to left and right outputs to effect steering (sometimes referred to as "steer-over"). In particular, this invention pertains to that class of multiple range steering transmissions which also have distinctly different speed and torque ratios in the various ranges without having steer reversal, i.e., with "steer positive" steering and without having clutches in the steer path. In this respect, the speed and torque ratios referred to are understood to be the ratios existing between the output from the included hydrostatic component and the transmission output with the speed and torque ratios being related as inverse functions. Measurement is between the hydrostatic and final outputs, especially in a split output transmission, because the ratios are fixed between transmission input and the input to the hydrostatic component and infinite between the input and output of the hydrostatic component, i.e., within the hydrostatic component. A split output hydromechanical transmission as the terminology is established in the art (Tuck "Split Power Transmission,", U.S. Pat. No. 3,433,095) is one in which a plurality of power paths from input to output are joined by a final power combining unit. In the split output example, the combining unit receives power in one path directly from transmission input and in another path from input via an included hydrostatic unit as opposed to a split input (Polak, "Input-Split Hydromechaniced Transmission"U.S. Pat. No. 3,982,448).
2. Description of Prior Art
The transmission of Polak, U.S. Pat. No. 3,596,535, is an example of synchronous shifting multi-range hydromechanical steering transmissions using dedicated hydrostatic components for steering, i.e., a "steer-over" transmission. Polak is of particular interest because the different speed and torque ratios in different ranges are shown by the slope of the lines in FIG. 2 of that patent. The transmission of Tuck, U.S. Pat. No. 3,590,658, is one example of synchronous shifting multi-range hydromechanical steering transmissions using internal steer. However, this transmission is really two coordinated truck (single output) transmissions which have clutches in the steer path and require coordination of range shifts which is in turn complicated by the fact of steer reversal. The transmission of Polak, U.S. Pat. No. 3,538,790, is another example of a synchronous shifting multi-range hydromechanical steering transmission having integral steer. It is full reversing, has a hydrostatic low range and a hydromechanical high range and has two mirror-image halves (i.e., two joined truck transmissions). The transmission disclosed in U.S. Pat. No. 3,815,698 (Reed) is a more pertinent example of a synchronous shifting multi-range hydromechanical steering transmission having positive internal steering. This transmission which is a predecessor model to the transmission forming the subject matter of this application has a hydrostatic reverse and first forward range and hydromechanical second and third forward ranges in which the speed and torque ratios, as defined above with respect to the field of invention, of second range are different from that of the first and third ranges which are identical. A four range modification of that transmission has been built and tested in which the fourth range has the same speed and torque ratios as second range and has the same relationship to third range as the second range has to the first and reverse range. The transmission disclosed in U.S. Pat. No. 3,861,240 (Nolan and Reed) is a synchronous shifting multirange hydromechanical truck transmission having hydromechanical reverse, first, second and third ranges which as stated therein (Column 8, lines 4-14) and illustrated in FIG. 2, provides a different and distinct torque ratio between the included hydrostatic transmission and the output in each forward range. (The torque ratio of reverse is the same as first range.) No infinitely variable hydromechanical steering transmission with internal steering (whether or not positive) having a different and distinct torque ratio in at least three forward ranges has been identified in the prior art.